We present the results of a deep wide-field near-infrared survey of 12 square degrees of the Pleiades conducted as part of the UKIDSS Deep Infrared Sky Survey ( UKIDSS ) Galactic Cluster Survey ( GCS ) .
We have extracted over 340 high probability proper motion members down to 0.03 solar masses ( M _ { \odot } ) using a combination of UKIDSS photometry and proper motion measurements obtained by cross-correlating the GCS with data from the Two Micron All Sky Survey ( 2MASS ) , the Isaac Newton ( INT ) and the Canada-France-Hawai ’ i ( CFHT ) telescopes .
Additionally , we have unearthed 73 new candidate brown dwarf members on the basis of five band UKIDSS photometry alone .
We have identified 23 substellar multiple system candidates out of 63 candidate brown dwarfs from the ( Y - K , Y ) and ( J - K , J ) colour-magnitude diagrams , yielding a binary frequency of 28–44 % in the 0.075–0.030 M _ { \odot } mass range .
Our estimate is three times larger than the binary fractions reported from high-resolution imaging surveys of field ultracool dwarfs and Pleiades brown dwarfs .
However , it is marginally consistent with our earlier “ peculiar ” photometric binary fraction of 50 \pm 10 % presented in , in good agreement with the 32–45 % binary fraction derived from the recent Monte-Carlo simulations of and compatible with the 26 \pm 10 % frequency recently estimated by .
A tentative estimate of the mass ratios from photometry alone seems to support the hypothesis that binary brown dwarfs tend to reside in near equal-mass ratio systems .
In addition , the recovery of four Pleiades members targeted by high-resolution imaging surveys for multiplicity studies suggests that half of the binary candidates may have separations below the resolution limit of the Hubble Space Telescope or current adaptive optics facilities at the distance of the Pleiades ( a \sim 7 AU ) .
Finally , we have derived luminosity and mass functions from the sample of photometric candidates with membership probabilities .
The mass function is well modelled by a log-normal peaking at 0.24 M _ { \odot } and is in agreement with previous studies in the Pleiades .